`IntoRawMode` only works with stdout

Although the IntoRawMode trait looks like it supports any writer: impl Writer, in fact it only works with stdout. The issue comes down to using sys::unix::{get_terminal_attr, ..} that have libc::STDOUT_FILENO hardcoded into them.

A solution would be update RawTerminal and IntoRawMode so it would be implemented for all impl Writer + AsRawFd. The raw fd needs to be retrieved from the writer and given to subsequent functions. I.e. something like that:

pub trait IntoRawMode: Write + AsRawFd + Sized {
    /// Switch to raw mode.
    ///
    /// Raw mode means that stdin won't be printed (it will instead have to be written manually by
    /// the program). Furthermore, the input isn't canonicalised or buffered (that is, you can
    /// read from stdin one byte of a time). The output is neither modified in any way.
    fn into_raw_mode(self) -> io::Result<RawTerminal<Self>>;
}

impl<W: Write + AsRawFd> IntoRawMode for W {
    fn into_raw_mode(self) -> io::Result<RawTerminal<W>> {
        let fd = self.as_raw_fd();
        let mut ios = get_terminal_attr(fd)?;
        let prev_ios = ios;

        raw_terminal_attr(&mut ios);

        set_terminal_attr(fd, &ios)?;

        Ok(RawTerminal {
            prev_ios: prev_ios,
            output: self,
        })
    }
}

Unfortunately, that would require a major bump in library version as it changes API. As a workaround until v3 it's possible to provide a second module raw2 that would properly handle writers that are not stdout, for example.

Let me know if you're interested in contribution!